Silica is particularly useful in reinforcing elastomers such as rubbers and in improving the resistance of rubbers to abrasion. They are also used as a carrier for dentally active substances, which are stored at the site of action and then release the active substance in small doses over a relatively long period of time (deposition effect, controlled release). The silica thus act as active substance stores which contain the active substance in absorbed, adsorbed or chemisorbed form. Any form of silica e.g., precipitated silica or silica gels or pyrogenic silica, maybe used. Silica are also used in particular for catalysis, inks and paper, in the food industry.
In catalysis, silica is used as a catalyst support, or as a porous layer coated or impregnated on monolithic supports. Due to its optical whiteness and opacity, silica is used as an inorganic charge in papers and also specialty papers. Silica is used in paper because of its porosity, which facilitates ink absorption. More particularly in the field of anal feeds, silica is used as a result of its absorption properties as a feed support, particularly as a support for methionine, vitamins, particularly vitamin A and E, for sucroglycerides, etc.
It is remarkable that precipitated silica of such widely different structures and properties can be made simply by mixing acid and sodium silicate in different ways at suitable times and temperature. It requires knowledge or reaction conditions in hot solution that leads to the following processes                1. Nucleation of particles        2. Growth of particles to a desired size        3. Coagulation to form aggregates by control of pH and metal ion concentration        4. Reinforcement of the aggregate structure to the desired degree without further nucleation.        
U.S. Pat. No. 4,243,428 discloses a process wherein precipitated silica is obtained by neutralization of a solution of sodium silicate with an acid under conditions which influence the properties of the silica finally obtained. The products obtained have a specific surface area ranging between 100 and 600 m2/g, which do not vary upon drying, due to the action of a strong acid on an alkaline silicate. The addition of strong acid to the alkaline silicate is interrupted one to three times. According to another embodiment of this prior art, the strong acid is added to the silicate solution in two phases wherein the temperature of the second phase is marginally higher than the first phase and both the phased were interrupted by a specific time factor. Also According to another embodiment of this prior art the temperature of the reacting medium is varied during two phases in which strong acid is successively added to the silicate, separated by one interruption. Another embodiment of this prior art is the speed at which the acid is added. According to a preferred embodiment of this prior invention to obtain silica having particle size less than about 20 μm, the acid is added to the silicate at a rate varying according to a law such that the residual alkalinity and the concentration of the total silica decrease linearly as a function of the reaction time, in accordance with the following equation in which:
  D  =                    Vi        ⁡                  (                                    Ci              /              62                        +                          Ca              /              98                                )                    ⁢              Ci        /        62                            T        ⁡                  (                                    Ci              /              62                        +                          Ca              /              98                        -                          Ci              ×                              t                /                62                            ×              T                                )                    2      D=flow at any time t; Vi=volume of initial silicate and; Ci=Na2O concentration of the initial silicate expressed in g/liter; Ca=concentration of the acid used, expressed in g/liter; T=total time of the reaction; t=time.
The drawback of this process is that the addition of acid is intermittently interrupted for at least three times and this rate of addition is to be controlled by a number of variables shown in the equation above. Besides, as per this equation the rate of addition of an acid is continuously increased during the course of the neutralization process which is critical for obtaining good quality silica. This makes the process complex. Moreover, control of temperature at two different phased at variable rate of addition is difficult.
U.S. Pat. No. 4,495,167 discloses a process to prepare precipitated silica having specific surface area higher than 400 m2/g, Dibutyl phthalate number higher than 300 and particle size having particles <63 μm. The process imbibe simultaneous addition of dilute sodium silicate and acid to water kept in the reaction vessel under continuous stirring and at 40 to 42° C. and maintaining the pH between 6 to 7. In this process, both the additions are stopped after 13th minutes and the addition interrupted for 90 minutes and again resumed till the completion of the precipitation. The entire reaction mass is then properly dispersed using a turbine type shearing device. Prior to filtration, the reaction mass is aged at room temperature for a period in the range of 12–17 hours. The drawback of the this process is that (i) the addition rates are to be critically controlled to maintain the pH between 6 to 7; (ii) need of high shearing and (iii) aging the mass for 12–17 hours make the process economically unviable.
U.S. Pat. No. 5,094,829 discloses a process to prepare reinforced precipitated silica having BET specific surface in the range of 220 to 340 m2/g with pore diameter of ca. 9 to 20 nm. The process involves the neutralization of alkali metal silicate solution with acid in multiple stages, viz. (i) initial neutralization of 60% of the total alkali at ca. 30 to 40° C.; (ii) simultaneous addition of alkali metal silicate solution and acid to neutralize 60% of equivalent alkali at 80 to 85° C. for minimum period of 115 minutes; (iii) adding acid to lower the pH below 7 for two times (iv) aging the reaction mixture for at least three times in the entire process. The drawback associated with this process is that the multiple stages of neutralization, increasing the pH, lowering the pH with acid make the whole process complex. Besides, the entire process involves aging of the reaction mixture at three different stages for a pretty long time. This makes the process uneconomical.
U.S. Pat. Nos. 5,034,207 and 5,123,964 disclose processes to prepared silica with BET specific surface in the range of 150 to 350 m2/g; bulk density between 60 to 120 g/l, and particles where at least 70% particles are from 1 to 6 μm. In this process, prior to neutralization of silicate solution, the later is heated to 70° to 80° C. and acid addition was continued till 50% of alkali present was neutralized and addition of the acid was interrupted for 30 to 120 minutes. Remaining acid was added till pH of suspension was around 3.0. The entire reaction mass is sheared at high speed for uniform dispersion. The silica suspension is optionally diluted with water and the coarse silica was separated by hydro-cyclone. The drawback of the process is that the addition of acid is interrupted for a very long period and reaction mass requires high shear which may adversely affect economics of the process.
U.S. Pat. No. 6,180,076 describes a process to prepare precipitated silica having BET specific surface in the range of 120–200 m2/g, Dibutyl phthalate index 120–300 and some of the particles are of less than 1 μm after degradation of the particles by ultra-sonication. Here silica is obtained by the reaction of alkali metal silicate with mineral acid at temperature 65° to 95° C. at pH of 7.0 to 11.0 with continuous stirring, the reaction is continued up to solids concentration of 40 g/l –110 g/l, the final pH is adjusted to a value between 3 and 5. The over all reaction is completed in two steps: 1) Addition of water glass and acid for 15 to 25 minutes followed by interruption of the addition for 30 to 90 minutes. 2) Addition of water glass and acid for 50 to 70 minutes. Total reaction time is 130 to 140 minutes. The drawback associated with this process is the controlled rate of addition to maintain the pH of the reaction mixture and the overall reaction time is 2 to 2.5 hours make the process unattractive.
U.S. Pat. No. 5,342,598 describes a process wherein the silica particulates are prepared by simultaneous addition of sodium silicate and a diluted acid into a dispersion of colloidal silica, under continuous agitation. At the completion of the reaction the pH of the suspension is between 3 and 7. In this process, colloidal silica is used as a nucleating agent and is separately prepared by heating a sodium silicate solution at around 75° C. and adding acid until the final pH is around 9.5. The drawback of the process is that both the solutions are to be added simultaneously and their rates are to be controlled very critically. Moreover, dilute colloidal silica is needed for precipitation of silica.
U.S. Pat. No. 5,851,502 discloses that precipitated silica is prepared by is prepared by introducing water into a precipitation vessel, adding water glass until an alkali value is between 5–15 and simultaneously adding water glass solution and sulfuric acid until the pH of the reaction mass is around 8.5, interrupting precipitation for some time and then further continuing the neutralization with acid until the pH of the slurry is ca. 4. The drawback of the process is that both the solutions are to be added simultaneously and their rates are to be controlled very critically and spray drying is required for obtaining desired product, which required high energy input.
U.S. Pat. No. 6,214,912 describes a process for the preparation of silica including the reaction of a silicate of an alkali metal M with an acidifying agent, whereby a suspension of precipitated silica is obtained, and then the separation and the drying of this suspension, the said process being characterized in that the precipitation is carried out in the following manner: (i) an initial stock is formed comprising a part of the total quantity of the silicate of an alkali metal M involved in the reaction, the silicate concentration, expressed as SiO2, in the said stock being lower than 20 g/l, (ii) the acidifying agent is added to the said initial stock until at least 5% of the quantity of M2O present in the said initial stock is neutralized; (iii) acidifying agent is added to the reaction mire simultaneously with the remaining quantity of alkali metal silicate such that the consolidation ratio, that is to say the ratio of quantity of silicate added (expressed as SiO.sub.2)/quantity of silicate present in the initial stock (expressed as SiO2) is between 12 and 100. Throughout the stage (iii) the added quantity of acidifying agent is preferably such that 80 to 99%, of the added quantity of M2O is neutralized. Aging of the reaction mixture for 1 to 60 minutes, after the above mentioned simultaneous addition of silicate solution and acid, is carried out. Finally, after the precipitation, in a subsequent stage, an additional quantity of acidifying agent is added to the reaction mixture. This addition is generally made until a pH value of the reaction mixture of between 3 and 6.5, obtained. The drawback of this process is that during simultaneous addition of silicate and acid in stage (iii) in such a way that neutralization of the M2O is between 80 to 99% of the total M2O, which makes the process complicated.
US Patent Application No. 20030118500 discloses a process for preparation of precipitated silica, comprising the reaction of a silicate with an acidifying agent, whereby a suspension of precipitated silica is obtained, followed by separation and drying of this suspension, characterized in that: the precipitation is carried out in the following way: (i) an initial stock solution is formed containing at least some of the total amount of the silicate used in the reaction and at least one electrolyte, the concentration of silicate (expressed as SiO2) in the said initial stock solution being between 50 and 60 g/l (ii) the acidifying agent is added to the said stock solution until a pH of between 7 and 8.5 for the reaction medium is obtained, (iii) the acidifying agent is added to the reaction medium along with, where appropriate, simultaneously, the remaining amount of the silicate, an additional amount of acidifying agent is added to the reaction medium, preferably until a pH of between 4 and 6 is obtained in the reaction medium. The separation comprises a filtration and washing operation using a filter equipped with a means of compacting, a suspension having a solids content of less than 17% by weight is dried by spraying. The drawback of the process is that both the solutions are to be added simultaneously and their rates are to be controlled very critically and spray drying is required for obtaining desired product, which required high energy input.
Indian Patent No. 176707 discloses a process for preparation of precipitated silica. Precipitated silica is prepared at ambient temperature (using hydrochloric acid) by preparing aqueous solution of sodium silicate having 1 to 1.3 N Na+ ion concentration, adding 14–16% hydrochloric acid to the solution at constant rate over a period of 10–60 min under continuous stirring to bring down pH of resultant mixture to around 10.8, continuing addition of the same acid for 3–6 hour to bring down pH between 3–4 to obtain the precipitated silica separating, washing, drying and pulverizing the said separated silica by known methods. The drawback of this process is that for complete neutralization of alkali metal at ambient temperature takes very long time which makes process uneconomical. Besides the process has a imitation that of all the mineral acids, the process is feasible only with hydrochloric acid.
U.S. Pat. No. 6,702,888 describes a process for preparation of precipitated silica containing aluminum for their application in the reinforcement of the elastomers. The process involves addition of an acidifying agent, to a preheated sodium silicate solution containing 100 g SiO2/liter and less than 17 g electrolyte/liter while maintaining temperature ca. 80° C., until the pH of the slurry is between 7.0 and 8.0. At this pH value, the remaining sodium silicate solution and the acidifying agent are added simultaneously at controlled rate while critically controlling the pH between 7.0 to 8.0 To this slurry diluted acid is added to bring the pH less than 7. The silica so obtained is separated by filtration and washed and filter cake is re-dispersed in water and to this either organic or inorganic salt of aluminum (Aluminum compound A) and a base is added simultaneously or an aluminate of alkali metal is added and pH of the reaction mixture is adjusted between 7.2 to 8.6. To this slurry again an acidifying agent is added to bring down the pH of the slurry between 3.4 and 4.5 and the product is separated by filtration and wet cake is dried to obtain a product. The drawback of the process is that silicate and acid solutions are added simultaneously and their rates of addition are to be controlled very critically. Moreover, the addition of aluminum compounds after filtering, washing and re-dispersing make the process overall complex and complicated.
U.S. Pat. No. 6,468,493 describes a process for the preparation of precipitated silica. The preparation is carried out in following way: i) an initial stock solution is prepared containing some of the total amount of silicate used in the reaction and at least one electrolyte. The silicate concentration in terms of SiO2 in the said initial stock solution is in the range of 40 to 330 g/l and electrolyte concentration is in the range of 12 to 20 g/l. Acidifying agent is added to the stock solution until pH value of between 7 and 8.5 is obtained. To this solution an acidifying agent and remaining silicate solution is simultaneously added in such a way that the pH of the reaction mixture does not change. Subsequently an additional amount of acidifying agent is added to the reaction mixture to bring down the pH of the reaction mixture is kept in the range of 4.5 to 5.5. The temperature of the reaction medium is kept between 68° C. to 98° C. The drawback of the process is that initial addition of acid to bring down pH to below 8.5 is critical as it leads to probability of gel formation. Besides, in the simultaneous addition phase the rate are to be critically controlled to maintain the pH.
U.S. Pat. No. 6,800,267 describes a preparation of precipitated silica involving following steps; a) heating a mixture of water and sodium silicate at a temperature of from 70 to 86° C. and adding sulfuric acid until half of the sodium silicate is neutralized; b) aging the more for a time of from 30 to 120 minutes; c) adjust the pH of the mixture with sulfuric acid to a range of from 3.0 to 7.0, thereby precipitating the aluminum-doped silica; d) filtering the aluminum-doped silica from the mixture to form a filter cake and washing the filter cake; e) drying and/or grinding the washed filter cake, wherein an aluminum salt solution is metered into the mixture at step a) and/or step c), the precipitated aluminum-doped silica has a BET surface of more than 300 m.sup.2/g, an Al.sub.2 O.sub.3 content of from 0.05 to 0.25% by weight, and the aluminum is distributed uniformly in the aluminum-doped silica. Wherein at least one or more of steps a), b), and c) are carried out with shearing. The drawback of the process is that it requires shearing, which involves high energy input and reaction time is longer than that of present invention which, adversely affect the economy of the process.